1. Field of the Invention
The present invention relates to a distance detection apparatus used in an automatic focus control apparatus for a camera or the like and, more particularly, to an improvement in an active-type distance detection apparatus which obtains distance information by operating two signals.
2. Related Background Art
FIGS. 1 to 3 illustrate an example of an automatic focus control apparatus which has a conventional distance detection apparatus of the type described above. Referring to FIG. 1, a light beam from a projection element 1 passes through a projection lens 2, reflected by a surface of an object 3, passes through a light-receiving lens 4, and becomes incident on a photosensor 5. As can be seen from FIG. 2, the input surface of the photosensor 5 is divided into two photosensor portions 5aand 5b. The beam incident on the portions 5a and 5b are photoelectrically converted thereby, and distance information is obtained based on the outputs from the portions 5a and 5b. As indicated by the solid line in FIG. 1, when the light beam reflected by the object 3 becomes as a beam spot S (FIG. 2) at the center between the portions 5a and 5b, the respective portions 5a and 5b receive the same amount of light, as shown in FIG. 2a. When the object is far as indicated by 3', the portion 5a receives a large amount of light and the portion 5b receives a small amount of light, as shown in FIG. 2b. When the object is near as indicated by 3", the portion 5b receives a large amount of light and the portion 5a receives a small amount of light, as shown in FIG. 2c. In other words, when the two portions 5a and 5b receive the same amount of light, as shown in FIG. 2a, the in-focus state is detected. When the state shown in FIG. 2b is detected, the near-in-focus state is detected. When the state shown in FIG. 2c is detected, the far-in-focus state is detected. In the case of near- or far-in-focus state, the photosensor 5 (along the directions indicated by the arrows of FIG. 1) is moved in synchronism with the imaging lens so as to perform automatic focusing control.
The automatic focusing control operation will be described with reference to a block diagram shown in FIG. 3. The photosensor 5 is moved in synchronism with the projection lens 6. More specifically, when a driving motor 7 is driven, the photosensor 7 is moved through a cam and the like. The outputs of the portions 5a and 5b are respectively connected to sense amplifiers 8a and 8b, DC component eliminating high pass filters 9a and 9b, analog switches 10a and 10b as detectors, and integrating circuits 11a and 11b. A microcomputer 12 supplies pulse signals to the projection element 1 and the analog switches 10a and 10b through a driver 13. In response to the pulses, the projection element 1 generates pulse beams. The analog switches 10a and 10b are turned on only during the emitting periods of the projection element 1 to supply input signals A and B received from the sense amplifiers 8a and 8b through the high pass filters 9a and 9b to the integrating circuits 11a and 11b. The signals A and B are integrated by the integrating circuits 11a and 11b and the integrated outputs are supplied to an adder 14 and a subtractor 15, respectively. The adder 14 and the subtractor 15 calculate sum and difference signals (A+B) and (A-B) and supply them to the microcomputer 12. The microcomputer 12 discriminates if the imaging lens is in the in-focus state or the out-of-focus state (near-in-focus state or far-in-focus state) in accordance with whether the integrated value of the sum signal (A+B) has reached a predetermined value VH and with whether the integrated value of the difference signal (A-B) has exceeded a threshold value .+-.VD. More specifically, when the sum signal (A+B) has reached the predetermined value VH and the difference signal (A-B) has not exceeded the threshold value .+-.VD, the microcomputer 12 determines that the imaging lens is focused. When the difference signal (A-B) exceeds the threshold value .+-.VD before the sum signal (A+B) reaches the predetermined value VH, the microcomputer 12 determines that the imaging lens is out of focus. In the latter case, the microcomputer 12 supplies an automatic focus control signal N to the driving motor 7 to move the projection lens 6 and the photosensor 5 in predetermined directions. When the outputs from the portions 5a and 5b indicate substantially an equal input light amount after this automatic focus control, the movement of the projection lens 6 is stopped, i.e., the automatic focus control signal N is disabled.
As described above, in the conventional apparatus shown in FIG. 3, the signals A and B from the portions 5a and 5b are integrated at the same time, distance information is calculated in accordance with the two integrated values, and automatic focus control is performed in accordance with the calculation results. For this purpose, two circuits are required: a circuit for processing the signal A (from the sense amplifier 8a to the integrating circuit 11a) and a circuit for processing the signal B (from the sense amplifier 8b to the integrating circuit 11b). This results in a complicated and large circuit and two difference sets of circuit characteristics must be set and controlled (gain, offset voltage and the like).
The present applicant has previously proposed an apparatus wherein signals A and B are time-divisionally processed, i.e., one signal, e.g., the signal A is integrated by a known Miller integrating circuit for a predetermined time period t1 (FIG. 4), and reverse integration is performed using the sum signal (A+B), as per Japanese Patent Application Laid open No. 19116/1985. Although this apparatus results in a small circuit and need not have two sets of characteristics, it has the following problems. When the signal A is integrated for a time period t1, the integrated value is At1. When the sum signal (A+B) is integrated for a time period t2, the integrated value is (A+B)t2. This apparatus thus performs reverse integration for the time period t2 required that an output level VT of the Miller integrating circuit after the time period t1 becomes an initial level V0 (i.e., the apparatus performs reverse integration such that the two integrated values equal each other). The distance information is calculated in accordance with the ratio t1/t2. When this method is adopted in the conventional apparatus shown in FIG. 3 (of the type wherein the in-focus state is detected when an equal amount of light becomes incident on the portions 5a and 5b), assuming the fact that the light reflected by the object in the in-focus state becomes incident at the center of the photosensor, the in-focus state is detected when t2=t1/2. Therefore, the relation A=B can be confirmed from At1=(A+B)t2 and t2=t1/2. In integration processing of any kind, an offset voltage caused by integration of a small DC drift current is always involved. When a drift signal is represented by D, the above relation can be rewritten as (A+D)t1=(A+B-D)t2. Since the in-focus state is detected if t2=t1/2, a relation A=B-3D is obtained and this reveals that a distance detection error has occurred due to the offset voltage. In order to remove such an error, zero adjustment of the offset voltage must be performed with care. If the offset voltage changes due to temperature changes or the like after such zero adjustment is made, an automatic offset voltage adjustment circuit must be added to compensate for such changes in offset voltage.
In such an apparatus, the output signals from the portions 5a and 5b largely fluctuate in accordance with object conditions. More specifically, when the object distance is short and the object has a high reflectivity, the output signal levels from the portions 5a and 5b become high. When the object distance is far and the object has a low reflectivity, the output signal levels become low. When the output signal levels from the portions 5a and 5b largely fluctuate in this manner, the signals may fall outside the dynamic range (the signal levels approach the power source voltage and saturate) of the circuit, and correct distance detection cannot then be performed.
In a conventional apparatus of this configuration, infinite distance is determined when the integrated values over a certain time period of the signals from the portions 5a and 5b are lower than an infinity threshold level. The output signals from the portions 5a and 5b are influenced by the reflectivity of the object. Therefore, even for objects at the same distance, if the projection spot is incident on an object portion having a low reflectivity, the resultant integrated values may be smaller than a predetermined value. In this case, the object at a detectable distance may be detected to be at an infinite distance.
In the apparatus previously proposed by the present applicant, distance information is obtained from the sum signal (A+B) and the signal A. Therefore, when a larger portion of the spot S is incident on the portion 5b, the level of the signal A becomes very low. In this case, the S/N ratio is low and distance information cannot be obtained with high precision.